Generic extending class with companion object in Kotlin

Question

I write the following code but it doesn't works because I get an unresolved reference error.

class Chromosome { constructor() {} companion object { fun newInstance(): Chromosome { return Chromosome() } } } class Population<T>(size: Int) where T: Chromosome { var population: Array<T> = Array(size, { _ -> T.newInstance() }) }

I want a generic type T that extends a certain class or interface. I need to call a static factory methods because I cannot write T() in Array class constructor. Are there any way to wrap Array class calling a default constructor?

I tried with a different approach but it still not works. This time I get an Cannot use 'T' as reified type parameter. Use a class instead error.

class Population<T>(size: Int, init: () -> T) where T: Chromosome { var population: Array<T> = Array(size, { _ -> init() }) }

This is as problematic in Java as in Kotlin. How would you implement this exactly in Java? — tynn
– tynn, Commented Oct 30, 2017 at 15:15
@tynn Good question. I tried to implement in Java but it doesn't work too. So you are right when you say that this is problematic in both languages. — Mpac
– Mpac, Commented Oct 30, 2017 at 16:36

tynn · Accepted Answer · 2017-10-30 18:35:39Z

The Array type acts the same as the array in Java. Therefore it is special and does not work well with generics. Instead you should use a Java List approach. This means to use MutableList in Kotlin.

class Population<T: Chromosome>(size: Int, factory: () -> T) { var population = MutableList(size, { factory() }) }

Also be sure to implement Chromosome as open or abstract class. Otherwise you wouldn't need to make the population generic and the Array approach would work again.

Jd Prajapati · Accepted Answer · 2017-10-30 15:18:05Z

You can try your Population class as code like below:

 class Population<T : Class<*>>(size: Int) where T: Chromosome { ... }

Using above code your Population class can able to get the generic type of class.

I hope it helps you.

Thank you for your suggestion but it doesn't work. I get these errors: Upper bounds of T have empty intersection and Only one of the upper bounds can be a class.

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